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26.1 - SOUND. Longitudinal wave of material objects because of vibrations Pitch = frequency High pitch = high freq. Range: 20 Hz – 20,000 Hz < 20 Hz = infrasonic > 20,000 Hz = ultrasonic. 26.2 – SOUND IN AIR.

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26 1 sound
26.1 - SOUND
  • Longitudinal wave of material objects because of vibrations
    • Pitch = frequency
    • High pitch = high freq.
  • Range: 20 Hz – 20,000 Hz
    • < 20 Hz = infrasonic
    • > 20,000 Hz = ultrasonic
26 2 sound in air
26.2 – SOUND IN AIR
  • The vibrating source “pushes” and “pulls” on the air causing alternating locations of compression & rarefaction
    • Compression – air molecules are squeezed together
      • Higher pressure
    • Rarefaction – molecules are spread apart
      • Lower pressure
26 3 media that transmit sound
26.3 – MEDIA THAT TRANSMIT SOUND
  • Sound travels through all materials
  • Speed of sound: solid > liquid > gas
  • Liquid is a better conductor of sound
26 4 speed of sound
26.4 – SPEED OF SOUND
  • Depends on temperature & mass of particles & elasticity
    • ↑ temperature = ↑ speed ~ 0.6 m/s per 1°C
    • Lighter particles move faster
    • The faster the material can change back to original shape (its elasticity) – the faster sound travels
26 5 loudness
26.5 - LOUDNESS
  • Sound intensity α A2
    • Threshold of hearing = 10-12 W/m2
    • Threshold of pain = 1 W/m2
      • This is a huge range
  • How loud something is ≠ intensity
    • Doubling loudness, ↑ intensity by 10
    • Use log (base 10) scale
  • Loudness has range: 0 – 12 bel (B)
  • A finer scale is used: the decibel (dB)
    • Range: 0 – 120 dB
26 6 natural frequency
26.6 – NATURAL FREQUENCY
  • All objects when disturbed will generate their own characteristic vibration
    • Depends upon: material (elasticity) & shape
  • This is the frequency to which the least amount of energy is required to continue the vibrations
26 7 forced vibrations
26.7 – FORCED VIBRATIONS
  • An object is made to vibrate because of vibrations of another object
  • Sounding boards in musical instruments make sound much louder
26 8 resonance
26.8 - RESONANCE
  • The frequency of vibration matches objects natural frequency
    • Causes an increase in amplitude
    • Swings, breaking step across bridges
26 9 interference
26.9 - INTERFERENCE
  • Sound waves can have constructive/destructive interference
    • Produces louder/quieter sound
    • In phase = loud, out of phase = quiet
    • Used in noise cancelling technology
26 10 beats
26.10 - BEATS
  • Periodic variation in loudness due to two tunes with small difference in frequency as they interfere
  • Beat frequency = difference between the two frequencies
  • Used to tune instruments
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